Fabrication of a total internal reflection optical switch with vertical fluid fill-holes

ABSTRACT

A method of fabricating a switching element or a matrix of switching elements includes providing a waveguide substrate having at least two waveguides that intersect at a trench such that optical coupling between the waveguides is dependent upon the presence or absence of an index-matching fluid at the intersection of the waveguides with the trench. Fluid is supplied to the trench via a fluid fill-hole that extends through a heater substrate in a direction that is generally perpendicular to a substrate surface on which at least one heater is fabricated. In the preferred embodiment, the fluid fill-hole is formed in a step of inductively coupled plasma (ICP) reactive ion etching (RIE). The waveguide substrate having at least two waveguides and the heater substrate having the heaters and the fill-hole are bonded together after the substrates are aligned such that the trench is in fluid communication with at least one fluid fill-hole and is in thermal communication with at least one heater. Optical fibers are then coupled to the waveguides. Preferably, a structurally weakened edge portion is formed during the ICP RIE step so that the edge portion can be removed after the two substrates are bonded, allowing uninhibited access of the optical fibers to the waveguides.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of Ser. No. 09/025,892, filed on Feb.18,1998, now U.S. Pat. No. 6,055,344.

TECHNICAL FIELD

The invention relates generally to optical switching elements and moreparticularly to methods of fabricating switches in which opticalcoupling among waveguides is determined by manipulating fluid.

BACKGROUND ART

While signals within telecommunications and data communications networkshave been traditionally exchanged by transmitting electrical signals viaelectrically conductive lines, an alternative medium of data exchange isthe transmission of optical signals through optical fibers. Equipmentfor efficiently generating and transmitting the optical signals has beendesigned and implemented, but the design of optical switches for use intelecommunications and data communications networks is problematic.

U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to theassignee of the present invention, describes a switching matrix that maybe used for routing optical signals from any one of a number of parallelinput optical fibers to any one of a number of parallel output opticalfibers. A functionally related matrix of switching elements is describedin U.S. Pat. No. 4,988,157 to Jackel et al. An isolated switchingelement 10 is shown in FIG. 1, while a 4×4 matrix 32 of switchingelements is shown in FIG. 2. The optical switch of FIG. 1 is formed on asubstrate. The substrate may be a silicon substrate, but other materialsmay be used. The optical switch 10 includes planar waveguides defined bya lower cladding layer 14, a core 16 and an upper cladding layer 18. Thecore is primarily silicon dioxide, but with other materials that affectthe index of refraction of the core. The cladding layers should beformed of a material having a refractive index that is substantiallydifferent from the refractive index of the core material, so thatoptical signals are guided along the core material.

The core material 16 is patterned to define an input waveguide 20 and anoutput waveguide 26 of a first waveguide path and to define an inputwaveguide 24 and an output waveguide 22 of a second waveguide path. Theupper cladding layer 18 is then deposited over the patterned corematerial. A trench 28 is etched through the core material and the twocladding layers to the substrate. The waveguides intersect the trench atan angle of incidence greater than the critical angle of total internalreflection (TIR) when the trench is filled with a vapor or gas. Thus,TIR diverts light from the input waveguide 20 to the output waveguide22, unless an index-matching material is located within the gap betweenthe aligned waveguides 20 and 26. The trench 28 is positioned withrespect to the four waveguides such that one sidewall of the trenchpasses through or slightly offset from the intersection of the axes ofthe waveguides.

In the 4×4 matrix 32 of FIG. 2, any one of four input waveguides 34, 36,38 and 40 may be optically coupled to any one of four output waveguides42, 44, 46 and 48. The switching arrangement is referred to as“non-blocking,” since any free input fiber can be connected to any freeoutput fiber regardless of which connections have already been madethrough the switching arrangement. Each of the sixteen optical switcheshas a trench that causes TIR in the absence of an index-matching liquidat the gap between collinear waveguides, but collinear waveguides of aparticular waveguide path are optically coupled when the gaps betweenthe collinear waveguides are filled with an index-matching fluid.Trenches in which the waveguide gaps are filled with fluid arerepresented by fine lines that extend at an angle through theintersections of optical waveguides in the array. On the other hand,trenches in which there exist an absence of index-matching fluid at thegaps are represented by broad lines through a point of intersection.

The input waveguide 20 of FIGS. 1 and 2 is in optical communication withthe output waveguide 22, as a result of reflection at the empty gap oftrench 28. Since all other cross points for allowing the input waveguide34 to communicate with the output waveguide 44 are in a transmissivestate, a signal that is generated at input waveguide 34 will be receivedat output waveguide 44. In like manner, input waveguide 36 is opticallycoupled to the first output waveguide 42, the third input waveguide 38is optically coupled to the fourth output waveguide 48, and the fourthinput waveguide 40 is coupled to the third output waveguide 46.

There are a number of available techniques for changing an opticalswitch of the type shown in FIG. 1 from a transmissive state to areflective state. In the above-identified patent to Jackel et al., wateror a refractive index-matching liquid resides within the gap betweenwaveguides until an electrochemically generated bubble is formed. A pairof electrodes is positioned to electrolytically convert the liquid togaseous bubbles. A bubble at the gap between collinear waveguidescreates an index mismatch and causes light to be reflected at thesidewall of a trench. The bubble can be destroyed by a second pulsehaving the appropriate polarity. Removing the bubble returns the switchto the transmissive state.

Japanese application No. 6-229802 of Sato et al. (Kokai No. 8-94866)describes the use of heaters to supply and remove index-matching liquidto and from a gap that is intersected by two waveguides. The flow ofliquid within a slit (i.e., trench) is controlled by selectivelyactivating heater elements. The index-matching liquid may be a lowviscosity silicon oil. Approximately one-third of the volume of the slitis filled with such liquid prior to bonding a surface cap substrate to asubstrate on which the waveguides are fabricated. The fixed quantity ofsealed liquid is manipulated by selectively activating one of two heaterelements. Activating a first heater element locates the sealed liquid atthe gap between two waveguides, while activating a second heater elementremoves the liquid from the gap between the waveguides.

If the liquid in the Sato et al. device were to be channeled to thetrenches from the edge of the switching device, the channels wouldrequire significant space on the device footprint. The waveguides, theheaters and the thin-film electrical connections to the heaters must beproperly mapped and fabricated. The concern is that the additionalrequirement of forming the channels would cause space requirements toexceed space availability. In an exemplary application, thecenter-to-center distance between parallel waveguides may be 250 μm inorder to match the pitch of conventional optical fiber ribbon cables.Design and production of liquid-feed channels having a sufficient volumefor ensuring proper operation of the switching elements would bedifficult in such an application. Another concern is that if theadditional channels are formed in the waveguide substrate, there may beadditional optical loss in the device.

Conversely, the concern with sealing the index-matching liquid withinthe trench, as taught by Sato et al., is that the volume of liquidcannot be adjusted after the device is fully assembled. That is, liquidcannot be added or removed if it is determined that the sealed volume isnot optimal.

What is needed is a fabrication method and a switching device thatprovide sufficiently large fluid fill-channels to trenches that areintersected by closely spaced waveguides, with the formation of thefluid feed-channels not impeding upon the process of fabricating theswitch structure.

SUMMARY OF THE INVENTION

A method of fabricating a switching element includes providing awaveguide substrate and a heater substrate, and includes forming atleast one fluid fill-hole through the heater substrate in a directiongenerally perpendicular to the major surfaces of the heater substrate.The waveguide substrate has at least two optical waveguides, includingfirst and second waveguides that intersect at a trench. The opticalcoupling between the first and second waveguides is dependent upon thepresence or absence of a fluid at the intersection of the waveguideswith a trench. With the completion of the fill-hole, heater andwaveguide structures, the two substrates are aligned and bonded, suchthat the trench is in fluid communication with at least one fill-holethrough the heater substrate and is in thermal communication with atleast one heater on the surface of the heater substrate.

In the preferred embodiment, there are dielectric layers on the oppositesides of the heater substrate prior to the steps of forming heaters andfluid fill-holes. The dielectric can be thermal oxide, such as silicondioxide (SiO₂). The heaters are formed on one surface of the heatersubstrate using fabrication techniques that are conventional to thefabrication of heaters for inkjet printheads. In addition to formingheaters, a conductive layer is patterned to provide electrical leadsfrom the heaters to bond pads for connection to heater drive-circuitry.While a single switching element may be formed using the invention, amatrix of switching elements is typically formed. In the preferredembodiment, each switching element is selectively switched from atransmissive state to a reflective state by creating and/or moving abubble within the trench. In the embodiment in which the bubble is movedinto and out of the gap between the ends of waveguides, there are twoheaters for each switching element. A first heater is positioned at thegap between the waveguide ends, and the other heater is aligned with thetrench that includes the gap, but at a distance from the first heater.Also, in the preferred embodiment, each trench is aligned with two ormore fluid fill-holes. By placing fluid fill-holes on both sides of thegap, bubble motion is more reliably controlled.

Forming the fluid fill-holes through either the heater substrate or thewaveguide substrate is problematic. Waveguide structures that comprisewaveguide layers on a silica substrate do not accommodate the use ofdrilling techniques to form fluid fill-holes that are sufficientlysmall. For waveguide structures on silicon substrates, conventional wetetching techniques (such as KOH or EDP) do not provide desired results,since waveguides may be only 250 μm apart but are fabricated oncommercial silicon wafers having a thickness of 500 μm to 1 mm. Sincethe slope of the wet-etched sidewalls is approximately 55°, the fluidfill-holes tend to merge horizontally before the vertical etching iscompleted. Thinner wafers may be used to decrease the vertical etchdimension, but the reduced thickness creates additional concernsrelating to handling, wafer bowing and breakage.

Drilling techniques may be used to penetrate the heater substrate withnear-vertical sidewalls. One technique is sand drilling, but the stateof the art with regard to this technique does not easily accommodate itsuse to form the fluid fill-holes. It is difficult to fabricate thenecessary metal “drill bit,” a part with a very narrow diameter holewith a 50:1 aspect ratio, to focus the sand spray. Another technique islaser drilling, that can fabricate holes with the required aspect ratio,but the heat generated by this operation may melt any circuitry within afew hundred microns of the fill-hole. This result is unacceptable inapplications in which the heaters and the conductive leads to theheaters are closely spaced on the front surface of the heater substrate.Both of these drilling processes are relatively slow because each holemust be drilled individually.

A drilling technique that operates well for forming the fluid fill-holesvertically through a substrate is referred to as inductively coupledplasma (ICP) reactive ion etching (RIE). ICP is based on the principleof generating a time-varying axial magnetic field to induce an electricfield which effectively confines plasma current. The process includesalternately applying an etch gas and a passivation gas. The anisotropicetching of silicon wafers using ICP RIE for forming microelectromechanical systems (MEMS) has been developed by STS Ltd (SurfaceTechnology Systems) of Wales, United Kingdom. This process has theadvantage of forming all the holes simultaneously.

As previously noted, there are preferably thermal oxidation layers onopposite surfaces of the heater substrate. One of the thermal oxidationlayers is patterned to define the positions of the fluid fill-holes,while the other thermal oxidation layer is used as an etch stop. The ICPRIE technique has a high selectivity in etching the substrate relativeto the thermal oxidation layer. The thermal oxidation etch-stop layer isleft intact, but with a reduced attachment. However, lateral etching(i.e., undercut etching) will occur after the vertical etching reachesthe heater surface, unless the ICP RIE step is terminated upon reachingthe thermal oxide layer at the exit. One concern is that if the etchdirection is from the back surface of the heater substrate to thesurface having the heaters and leads, lateral etching will remove thesubstrate material (e.g., silicon) supporting the circuitry near theholes. Since the etch rate is more rapid at the edge of the substratethan at the center, holes at the edge potentially undergo considerablelateral etching before holes penetrate to the thermal oxidation layer onthe exit side at the center of the substrate. Thus, the preferredembodiment is one in which the etch direction is from the heater surfaceto the back surface.

Following the formation of the fluid fill-holes, the heater substrate isbonded to the waveguide substrate. Liquid adhesives of the type that areappropriate for spin-on application are preferred to the thicker dryfilm adhesives that are often used in inkjet fabrication. The liquidadhesives provide a thinner bonding layer (5 μm or less) than the dryfilm adhesives (between 12 and 50 μm). Moreover, it is preferred to spinthe adhesive on the heater substrate, rather than the waveguidesubstrate, since (1) it is less likely to adversely affect the opticalproperties of the resulting device, (2) it is less tedious and morereproducible than applying the adhesive to the diced waveguidestructures typically used, (3) the waveguide structures are moreexpensive than the heater structures and, consequently, should not beused for process development that has less than an optimal yield, and(4) the spun film will be more uniform over the heater wafer, due to itssmaller variations in topology.

If the etch direction in the ICP RIE step is from the back surface tothe heater surface of the heater substrate, a thermal oxidation layer ispreferably left when the adhesive layer is applied. The thermaloxidation layer functions as a membrane over the fluid fill-holes duringthe application of the adhesive layer, thereby accommodating theformation of a uniformly thick adhesive layer. The membranes then can beeasily removed by rupturing with pressure from a gas gun or with anadditional etching step.

Another concern is that the ends of the waveguides at the edges of thewaveguide substrate may not be sufficiently exposed to obtain a couplingloss after the heater substrate has been bonded in position. In oneembodiment, the vertical distance from the center of a waveguide to anupper cladding layer is only 25 μm, while the distance from the centerof a conventional optical fiber to the outside of the cladding of thefiber is 62.5 μm. Thus, the fiber will tend to contact the heatersubstrate before reaching the edge of the waveguide. Preferably, theedge portion of the heater substrate is structurally weakened before thetwo substrates are bonded together. While individual waveguidesubstrates may be bonded to individual heater substrates, typically thewaveguide substrates are individually bonded to a wafer that includes anarray of “heater substrates.” The wafer is then diced to provide a batchof switching devices, each having a matrix of switching elements. Thestructurally weakened edge portion is removed after the bonding step.

In the preferred embodiment, the structural weakening is achieved duringthe ICP RIE step. A series of small, spaced through-holes, a continuousline, or a combination of small through-holes and one or more slots maybe formed to provide a perforation that defines the structurallyweakened edge portion. These smaller features are etched most of theway, but preferably not all of the way, through the wafer. The edgeportions can be subsequently removed without requiring a step of sawingthe edge portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an optical switching element that utilizes totalinternal reflection in accordance with the prior art.

FIG. 2 is a 4×4 matrix of switching elements of FIG. 1 to allowconnection of any one of a number of input waveguides to any one of anumber of output waveguides in accordance with the prior art.

FIG. 3 is a top view of a matrix of switching elements having opticalpaths that intersect trenches that are supplied with fluid in accordancewith the invention.

FIG. 4 is a top view of a switching element using bubble manipulationtechniques to switch between a reflecting state and a transmitting stateand having fluid fill-holes in accordance with the invention. It iscurrently in a reflecting state.

FIG. 5 is a top view of the switching element of FIG. 4 in atransmitting state.

FIGS. 6-16 are side sectional views that illustrate the fabrication of aswitching element in accordance with a first embodiment of theinvention.

FIGS. 17-29 are side sectional views that illustrate a second embodimentof fabricating a switching element.

DETAILED DESCRIPTION

With reference to FIG. 3, a matrix 50 of switching elements includes awaveguide substrate 52 that is attached to four fiber array substrates54, 56, 58 and 60. The waveguide substrate is preferably a silicon orsilica chip, but other materials may be used.

The waveguide substrate 52 is shown atop a heater substrate 53. Theheater substrate is preferably formed of a material that accommodatesthe use of techniques for forming conventional features of an inkjetprinthead, such as heaters and interconnects that extend from theheaters to bond pads 55 at the edges of the heater substrate. The bondpads are then connected to heater drive circuitry, not shown, foroperating the heaters. As will be explained fully below, fluidfill-holes 57 are formed within the heater substrate to supply anindex-matching fluid to trenches 94 on the surface of the waveguidesubstrate 52. Acceptable materials for the heater substrate 53 includesilicon and silica.

The waveguide substrate 52 has a four-sided configuration. If thesubstrate is square and each waveguide is fabricated to extendperpendicularly from an edge of the substrate, each optical switchingelement will be substantially identical to the switching element 10shown in FIG. 1. However, the preferred embodiment is one in which theangles of incidence of the waveguide on the trench are in the range of45° to 60°.

While not shown in the top view of FIG. 3, the interface between thewaveguide substrate 52 and each fiber array substrate 54, 56, 58 and 60can be intentionally tilted at an angle, such as eight degrees. Thisintentional tilt is intended to decrease the magnitude of reflectionbetween waveguides on substrate 52 and optical fibers on substrates 54,56, 58 and 60.

In FIG. 3, the waveguide substrate 52 includes sixteen input/outputwaveguides 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90and 92. Additionally, there are twenty-four intermediate waveguides toincrease the flexibility of the switching arrangement. Typically, theoperations of the switching matrix that are of greatest concern are theoperations that determine the optical coupling of the waveguides 62-68with the waveguides 86-92. The remaining waveguides 70-84 may be usedfor add and drop operations or to connect to other switches to form alarger scale switching matrix.

As previously described with reference to FIG. 1, optical coupling amongwaveguides is dependent upon the presence or absence of index-matchingfluid at the gaps between waveguide ends. If the lowermost trench inFIG. 3 does not include index-matching fluid at the gap, any signalsfrom the waveguide 62 will be diverted by total internal reflection tothe waveguide 92. On the other hand, if the gap at the intersection ofwaveguides 62 and 92 is filled with index-matching fluid, the opticalsignal from waveguide 62 will propagate through the trench to the nexttrench. Depending upon the presence or absence of fluid, the inputsignal from waveguide 62 may be diverted to any of the four waveguides86-92 or propagate straight through to waveguide 84. This versatilityapplies to the other three waveguides 64, 66 and 68 that are adjacent towaveguide 62.

Each of the waveguides 62-92 is formed of a core material, such as SiO₂that is doped with Ge or TiO₂, to define a first refractive index.Cladding material may be primarily SiO₂, but with a different dopant,such as B₂O₃ and/or P₂O₅, to define a second refractive index differentfrom the first refractive index. Because of the difference in refractiveindices, optical signals are guided along the waveguide cores.

Each of the fiber array substrates 54, 56, 58 and 60 includes fouroptical fibers 96. The pitch of the fibers matches the pitch of theinput/output waveguides on the waveguide substrate 52. An acceptablepitch is approximately 250 μm, but other center-to-center distances maybe utilized, such as 500 μm. Ideally, the waveguides have the same crosssectional geometry as the optical fibers. However, it is not feasible tofabricate substrate waveguides having a circular cross section.Moreover, the 8 μm dimension of a conventional fiber is not necessarilyoptimal for waveguides for which optical signals must propagate throughfluid-filled trenches in order to pass from one waveguide to asubstantially collinear waveguide. It has been determined thattransmission losses across fluid-filled gaps are reduced by increasingthe cross sectional area of the waveguides. Thus, the waveguides 62-92may be adiabatically tapered from a relatively large cross sectionalarea at the interior ends at the trenches 94 to a smaller crosssectional area at the interface with the optical fibers 96. The crosssectional geometry at the interior end is selected to achieve efficientoptical coupling across the trenches, while the cross sectional geometryat the exterior ends is selected to achieve high coupling efficiency tothe optical fibers. In one embodiment, the adiabatic tapering is from a16 μm×8 μm cross sectional geometry to an 8 μm square cross sectionalgeometry. Alternatively, the adiabatic taper may be incorporated alongthe fibers 96, such as by using a thermally diffused, expanded-core(TEC) fiber. As is known in the art, TEC fibers have an expandingportion at the ends of the fibers.

U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to theassignee of the present invention and is incorporated herein, describesa number of alternative approaches to switching a switching elementbetween a transmissive state and a reflective state. For each switchingelement within a matrix, there may be one or more heaters that can beused to create and/or manipulate bubbles within a fluid in order toswitch the state of the element. One approach is illustrated in FIGS. 4and 5. Within this approach, there are two micro heaters 100 and 102that control the position of a bubble 104 within a fluid-containingtrench 106. The fluid within the trench has a refractive index thatsubstantially matches the refractive index of the core material of fourwaveguides 108, 110, 112 and 114. An acceptable liquid is a combinationof isopropyl alcohol and glycerol. Other acceptable liquids arecyclohexane, methylcyclohexane, and M-pyrol.

At the opposite end of the gap that is aligned with the micro heater 100are fluid fill-holes 116 and 118. The fluid fill-holes are preferablylarger than the width of the trench 106 in order to provide a lowresistance flow. However, the fluid fill-holes must fit between closelyspaced waveguides and leave sufficient space for patterning conductiveleads that connect the heaters 100 and 102 to heater drive circuitry. Afluid fill-hole diameter may be within the range of 70 μm to 100 μm, butthis is not critical.

In the operation of the switching element 120 of FIGS. 4 and 5, one ofthe heaters 100 and 102 is brought to a temperature sufficiently high toform a gas bubble. Once formed, the bubble 104 can be maintained inposition with a reduced current to the heater. In FIG. 4, the bubble islocated at the gap of the four waveguides 108-114. Consequently, aninput signal along waveguide 108 will encounter a refractive indexmismatch upon reaching the trench 106. Thus, the switching element is ina reflecting state in FIG. 4. The activation of the heater 100 pins thebubble at the gap, so that the reflecting state is maintained as long asthe heater is activated.

In FIG. 5, the heater 100 at the gap between the ends of the waveguides108-114 has been deactivated and the second heater 102 has beenactivated. The bubble 104 is attracted to the activated heater. Thisallows index-matching fluid to fill the gap at the intersection of thewaveguides. The switching element 120 is in a transmitting state, sincethe input waveguide 108 is optically coupled to the collinear waveguide112, and the input waveguide 114 is coupled to the collinear waveguide110 without crosstalk between the two waveguide paths.

One embodiment of a fabrication method for forming switching elements inaccordance with the invention will be described with reference to FIGS.6-16. In this embodiment, the fluid fill-holes are formed from the backsurface of the heater substrate, with the back surface being defined asthe surface that does not include the heaters. An alternative embodimentin which the fluid fill-holes are formed from the heater surface to theback surface will be described below, with reference to FIGS. 17-29.

With reference to FIG. 6, a first step is to provide a heater substrate122. As previously noted, the heater substrate is preferably formed of amaterial that facilitates use of conventional inkjet and integratedcircuit fabrication techniques. A preferred material is silicon, butthis is not critical. In FIG. 6, the opposed major surfaces of thesubstrate 122 have a thermal oxidation layer 124 and 126. As will beexplained more fully below, each of the thermal oxidation layers plays asignificant role in the patterning of the fluid fill-holes, as well asother functions. A preferred material is SiO₂, but other materials maybe used. Also in the preferred embodiment, the heater substrate 122 isdouble-side polished prior to formation of the thermal oxidation layers124 and 126.

In FIG. 7, two conductive layers 128 and 130 have been formed on theheater surface of the heater substrate 122. Acceptable materials are alower layer 128 of Ta₂N and an upper layer 130 of Au. The lower layerfunctions primarily as a resistive material for defining the heaters,while the upper layer is highly conductive and is patterned to defineleads for connecting the heaters to heater drive circuitry. Otheracceptable materials for forming the heaters include TaAl, W andpolysilicon.

In FIG. 8, the two conductive layers 128 and 130 have been uniformlypatterned to define a lead. The patterning may be accomplished usingconventional photolithographic techniques. In FIG. 9, the highlyconductive upper layer 130 has been patterned to expose a region 132 ofthe more resistive lower layer 128. This exposed region is the heaterregion that generates thermal energy when electrical current is causedto conduct through the region from the patterned upper conductive layer130.

Referring now to FIG. 10, a passivation layer 134 is formed on theheater surface of the heater substrate 122. The passivation layerprovides electrical isolation of the two conductive layers 128 and 130from the index-matching liquid during operation of a switching element.The passivation layer may be SiC_(x), SiC_(x)+Si_(x)N_(y), Si_(x)N_(y),or a similar material. In FIG. 11, the passivation layer has beenpatterned to expose a surface region 136 of the conductive layer 130.This surface region 136 provides a bond pad for subsequent connection tothe heater drive circuitry that is used in operating the heater atregion 132. The patterning of the passivation layer 134 also exposes aregion 138 at which a fluid fill-hole will be formed.

In FIG. 12, the back side of the heater substrate 122 has been spun witha thick photoresist 147. The photoresist and the thermal oxidation layer126 have been patterned to expose a first region 140 that is alignedwith the region 138 and to expose a second region 142. The exposedregion 140 provides a means for forming the fluid fill-hole. The region142 provides a means for forming one of a series of holes and/or anarrow continuous slot to structurally weaken the edge portion of thesubstrate. That is, the series of holes (or the narrow slot) providesperforations for subsequently removing the left edge of the substrate,as viewed in FIG. 12.

The fluid fill-holes are formed in a direction perpendicular to themajor surfaces of the heater substrate 122. Thus, the supply of theindex-matching fluid does not significantly interfere with the thin filmelectrical connections to the heater region 132. However, it is somewhatmore problematic to form the fluid passages through the heatersubstrate, than it is to form fluid trenches along the surface of asubstrate. Conventional wet etching techniques may not be used in manyapplications, since the waveguides are only 250 μm apart and standardfour-inch commercial silicon wafers (i.e., 100 mm wafers) are on theorder of 500 μm thick. Because the slope of side walls formed usingconventional wet etching is approximately 55°, the fluid fill-holeswould merge horizontally before vertical etching is completed. Usingthinner wafers to decrease the vertical etch dimension is possible, butthe thinner wafers are more difficult to handle and more susceptible tobreakage.

Three drilling techniques for forming the fluid fill-holes wereconsidered. Sand drilling is an option, but there is an extremedifficulty of fabricating the necessary metal “drill bit,” a part with avery narrow-diameter hole with a 50:1 aspect ratio, to focus the sandspray. Another option was to use laser drilling to fabricate the fluidfill-holes, but the heat generated by the process can melt the thin filmcircuitry on the heater surface of the wafer, if the thin film circuitryis within a few hundred micron radius of the laser-drilled hole. Thus,laser drilling is not possible in some applications of the invention.The technique that is suitable is referred to as inductively coupledplasma (ICP) reactive ion etching (RIE). This technique is utilized inthe fabrication of micro electromechanical systems (MEMS) onsemiconductor wafers. A problem with the sand drilling and laserdrilling techniques is that the drilling processes are relatively slow,since each fluid fill-hole must be drilled individually.

There are a number of factors that are interrelated in the selection oftechniques for implementing the steps that are illustrated in FIGS.13-16. Merely forming the fluid fill-holes and the perforations is notsufficient. It is also necessary for large-scale manufacturing toprovide an adhesive layer for bonding the heater and waveguidesubstrates together, in addition to fabricating the fluid fill-holes.Implementing hole fabrication technologies may degrade the quality ofthe adhesive layer and/or the flatness of the adhesive layer, ifexecuted after the adhesive layer is applied. Moreover, switches may bedamaged if the holes are formed after the waveguide chip is bonded tothe heater substrate. Roller-coating the adhesive layer onto the surfaceof the heater substrate 122 is an option, but the result is typicallyless uniform than a spin-coated layer. On the other hand, the presenceof the fluid fill-holes renders it difficult to obtain a uniform coatingby spinning. In the embodiment of FIGS. 12-16, the problem is overcomeby using the thermal oxidation layer 124 as an etch-stop layer.

Thus, the thermal oxidation layer 124 on the heater surface of theheater substrate 122 serves a number of functions. The thermal oxidationlayer acts as a heat barrier in isolating the heater at region 132 fromthe substrate material. Moreover, the ICP RIE process is highlyselective with respect to etching silicon over silicon dioxide. If thethermal oxidation layer 124 is SiO₂, the vertical etch will terminateupon reaching the thermal oxidation layer 124. Referring to FIG. 13, thephotoresist has been removed from the back-side SiO₂ layer 126, but amembrane 144 of the front-side SiO₂ layer 124 remains to support theadhesive layer 146 that is applied as shown in FIG. 14. Leaving an SiO₂layer intact has the additional advantage of maintaining separationbetween the reactive gases that are used to etch the exposed face of thesemiconductor substrate 122 and the gas cooling (such as helium) incontact with the opposite face, i.e., the heater surface of the wafer.With a process that would open holes all the way through the wafer, thecooling gas would leak into the main etching chamber. Gas cooling hasadvantages over liquid cooling, and the substrate must be cooled duringa long etch in order to preserve the side wall quality.

In FIG. 14, the adhesive layer 146 is applied. While other materials maybe used, the adhesive layer may be a photoimageable polymer, such as aphotoimageable benzocyclobutene (photo-BCB). The adhesive layer 146 isapplied to the upper surface of the heater substrate 122 using a spin-onprocess. Since a thin adhesive layer (e.g., less than or equal to 5 μm)is preferred, liquid adhesive appropriate for spin-on application ispreferable to the thicker dry film adhesives (12-50 μm) that arestandard in many inkjet fabrication processes. It is not preferable tospin the adhesive layer on the waveguide substrate for four reasons: (1)the optically reflecting interface may be contaminated by applying theadhesive layer onto the interface; (2) the waveguides are oftenpurchased from a vendor after being sawed from a wafer form to a chipform, and it would require tedious and less reproducible adhesiveapplication if the layer were applied to many small parts; (3) thewaveguide structures are more expensive than the heater structures, andconsequently should not be used for a process development which has aless than optimal yield; and (4) the spun film will be more uniform overthe heater wafer, due to its smaller variations in topology.

Still referring to FIG. 14, after the ICP RIE step is completed, theadhesive layer 146 is spun onto the front surface of the heatersubstrate 122, including the thermal oxidation membranes 144 over thefluid fill-holes 148. In FIG. 15, the adhesive layer 146 is patterned toonce again expose the bond pad 136, the heater region 132 and the region138 that is aligned with the fluid fill-hole 148. The membrane 144 canthen be easily removed by rupturing with pressure from a gas gun or byusing a conventional etch step. The resulting structure is shown in FIG.16, with a waveguide substrate 150 bonded to the heater substrate 122.While FIG. 16 is not intended to be a drawing that is drawn to scale,and while the embodiments of FIGS. 3 and 16 are not completelycompatible, it is instructive to correlate FIGS. 3 and 16. The heatersubstrate 122 and the waveguide substrate 150 of FIG. 16 compare to theheater substrate 53 and the waveguide substrate 52 of FIG. 3. The bondpad 136 may be the center bond pad 55 in the lowermost array of bondpads 55 in FIG. 3. Thus, the bond pads 136 and 55 are exposed forconnection to heater drive circuitry. Along the vertical line from bondpad 55 to the intersection of waveguides 62 and 92 in FIG. 3, theintersection of the axes of these two waveguides 62 and 92 may be thelocation of the heater region 132 in FIG. 16. That is, the lowermosttrench 94 in FIG. 3 is similar to the position of the a trench 152 thatis aligned with the heater region 132 in FIG. 16.

Again referring to a vertical line extending upwardly from theintersection of the waveguides 62 and 92 in FIG. 3, the next feature ofthe switching matrix 50 of FIG. 3 is the intersection of a center fluidfill-hole of the trench 94 above the lowermost trench. This fill-holecompares to the fluid fill-hole 148 of FIG. 16 and the trench 94compares to the trench 154 of FIG. 16. The remainder of the verticalcross section through the switching matrix 50 is not shown in FIG. 16.

For ease of fabrication, it is desirable to avoid further sawing of thewaveguide substrates after they have been diced. However, the substrateitself must be even with or recessed behind the edges of the waveguides62-92 of FIG. 3, so that the optical fibers 96 can be efficientlycoupled to the waveguides to obtain low insertion loss. The difficultyis that the distance from the center of the waveguides to the uppercladding of the waveguides is only 25 μm, while the distance from thecenter of an optical fiber to the outside of the cladding is 62.5 μm.Thus, the fiber may hit the heater substrate 53 before reaching the edgeof the waveguide. In order to provide a recess without sawing thewaveguide substrate, waveguide chips may be bonded to heater chips aftersawing the heater chips (since the saw path is a straight line).Alternatively, “knockout” edge portions of the heater chips may beremoved after the bonding of the waveguide chips to a wafer array ofheater chips. While bonding chip-to-chip is possible, it is lessdesirable than the second alternative, since (1) it requires individualdie-to-die alignment, rather than die-to-wafer alignment, (2) itrequires protection of any spun-on adhesive during sawing of the heaterwafer, and (3) it is inherently more expensive. Thus, the “knockout”approach allows for less expensive large-area alignment. As a thirdalternative, the wafer can be sawed into strips of heater chips beforethe bonding process, thereby reducing the amount of thermal cycling anygiven device must undergo during the bonding of neighboring devices.

Returning to FIG. 12, the region 142 of the back surface of the heatersubstrate 122 that is exposed by patterning the back side thermaloxidation layer 126 may be used to define a structurally weakened edgeportion of the heater substrate. While not shown in FIG. 12, the region142 is only one within a series of such regions. During the ICP RIEstep, the substrate material is etched to form a series of holes, suchas the hole 156 shown in FIG. 13. The holes 156 have smaller lateraldimensions than the fluid fill-holes 148, and are aligned to define thestructurally weakened edge portion 158 of the substrate 122. The seriesof holes (or slot) forms a perforation or “dotted-line” along the edgeportion 158. While not shown in FIG. 13, the hole 156 preferably doesnot extend through the entirety of the substrate material 122. However,the perforations may extend through the entirety of the substratematerial, as long as sections of the wafer beyond these perforationsremain intact to prevent wafer breakage. It is possible to etch theperforations (or slot) while completing the etching of the fluidfill-hole 148, since the narrower perforations etch at a slower ratethan the larger fluid fill-holes.

Referring now to FIG. 16, after the steps of bonding the waveguidesubstrate 150 to the wafer that includes the heater substrate shown inFIG. 16, the heater substrate is sawed into discrete die. Thestructurally weakened edge portion 158 of FIG. 15 is then broken alongthe perforations formed by the series of holes 156. The removal of thestructurally weakened edge portion 158 provides access of optical fibersto the waveguides on the waveguide substrate 150. For purposes ofillustration, a dashed line 159 in FIG. 3 has been included to indicatethe edge portion that was removed from the bottom heater substrate 53 toaccommodate access of the fibers 96 on substrate 60 for coupling towaveguides 62-68.

In the formation of the structurally weakened edge portion 158, thewafer mask for defining the regions 142 of FIG. 12 should be designed sothat there are no connecting holes between different devices, therebyreducing the likelihood that an accidental cleave of the wafer willoccur along the line defined by the perforations. Otherwise, the waferremains structurally intact and does not break along the perforationsduring the dicing operation.

Again referring to FIG. 13, a concern with the execution of the ICP RIEstep is that the strong selectivity of etching the substrate material122 to the SiO₂ thermal oxidation layer 124 may cause lateral etchingonce the fluid fill-hole 148 reaches the thermal oxidation layer 124.The lateral etching could adversely affect the operation of theswitching element. FIGS. 17-29 illustrate an alternative embodiment tothe fabrication of a switching element. In the method according to FIGS.17-29, the fluid fill-holes and the 35 holes that define thestructurally weakened edge portion of the heater substrate are formedfrom the heater surface toward the back surface of the substrate.Referring to FIG. 17, thermal oxidation layers 162 and 164 are formed onopposite major surfaces of a wafer 160, such as a silicon wafer. Theprocess and the considerations are identical to those described withreference to FIG. 6.

A resistive heater layer 166 and a highly conductive lead layer 168 areformed on the heater surface of the heater substrate 160. The heater andlead layers are patterned using known techniques, such asphotolithography. The lead layer 168 is patterned to expose a region 170of the more resistive heater layer 166, so that current conductingthrough the lead layer must enter the heater layer at region 170,thereby causing thermal energy to be generated within this region.

In FIG. 18, a passivation layer 172 has been uniformly deposited acrossthe heater surface of the heater substrate 160. The passivation provideselectrical isolation and chemical protection of the two conductivelayers 166 and 168 from the index-matching liquid that is used duringoperation of the switching element. The passivation layer may beSiC_(x), SiC_(x)+Si_(x)N_(y), Si_(x)N_(y), or a similar material.

In the step illustrated by FIG. 19, the passivation layer 172 ispatterned to expose a region 174 for forming the fluid fill-hole, aregion 176 for the bond pad, and a region 178 for forming one of theholes that defines the structurally weakened edge portion of thesubstrate 160.

In FIG. 20, double-sided photoresist protection is provided by layers180 and 182. The top side photoresist layer 180 is used in aphotolithographic process to etch the thermal oxidation layer 164,re-exposing the fluid fill-hole region 174 and the perforation holeregion 178, as shown in FIG. 21. The front-side photoresist layer 180 isthen removed to provide the assembly shown in FIG. 22.

In FIG. 23, an adhesive layer 184 is applied to the heater surface ofthe heater substrate 160. Preferably, the adhesive layer is aspin-coated layer of a photoimageable polymer (e.g., photo-BCB).However, other materials and/or other application techniques may beutilized. In FIG. 24, the adhesive layer is patterned to expose theheater region 170, the fill-hole region 174, the bond pad region 176 andthe perforation region 178. Conventional techniques for patterning anadhesive layer may be utilized. Next, a photoresist layer 186 is appliedto the heater surface of the substrate 160 to protect the photoimageablepolymer during the ICP RIE process for forming the fluid fill-hole andthe perforation. The photoresist layer 186 is patterned, as shown inFIG. 25, to expose the regions to be etched during the ICP RIE process.

The ICP RIE etch step occurs from the heater surface side of the heatersubstrate 160 and is therefore somewhat more relaxed than the etch inthe opposite direction. Lateral etching of the substrate material uponreaching the thermal oxidation layer may improve performance of theswitching element when the lateral etching takes place at the side ofthe substrate from which the index-matching liquid is supplied.Referring now to FIG. 26, a fluid fill-hole 188 and a perforation 190are formed during the ICP RIE step. Both of the holes 188 and 190include expanded ends 192 and 194 as a result of lateral etching whenthe vertical etch reaches the back-side thermal oxidation layer 162.

In the next step, the back-side photoresist 182 and thermal oxidationlayer 162 are removed to provide the assembly shown in FIG. 27. Theperforation that includes the hole 190 defines a structurally weakenededge portion 196. Referring now to FIG. 28, the photoresist layer isthen removed to expose the heater region 170 and the bond pad region176, as well as the patterned adhesive layer 184.

In FIG. 29, a waveguide substrate 198 is bonded to the heater substrate160 and the photoimageable adhesive layer 184 is cured. The heaterregion 170 is aligned with a first trench 200 and the fluid fill-hole188 is aligned with an adjacent trench 202. In a final step, thestructurally weakened edge portion, not shown in FIG. 29, is removed toaccommodate the optical coupling of waveguides to optical fibers (notshown). The removal of the structurally weakened edge portion preferablyoccurs after the heater wafer has been diced into a batch of theswitching matrixes 50 shown in FIG. 3.

In operation, the fluid fill-holes 57 of FIG. 3 may be connected to asingle source of index-matching fluid. An advantage of the design isthat the trenches 94 readily fill with fluid when the waveguide chip 52is atop the heater chip. The switching matrix 50 is tested with thewaveguide chip in a position that allows the bubble motion to be viewedwith a standard visible-light microscope. If a puddle of index-matchingfluid is introduced underneath the matrix 50 from one side, the fluidfill-holes 57 wick up the fluid by capillary action and the trenches aresupplied with the fluid from the fill-holes by capillary action.Preferably, the trenches have the smallest cross sectional dimensionswithin the fluid supply path, so that capillary action provides areliable fluid supply.

What is claimed is:
 1. A method of fabricating a switching element inwhich fluid is manipulated so as to toggle said switching elementbetween a transmissive state and a reflective state, said methodcomprising the steps of: providing a substrate; and utilizinginductively coupled plasma (ICP) reactive ion etching (RIE) toselectively remove material from said substrate.
 2. The method of claim1 wherein said step of providing said substrate includes providing oneof a waveguide substrate for a pattern of optical waveguides and aheater substrate for a pattern of thermal heaters to selectivelymanipulate said fluid.
 3. The method of claim 1 wherein said step ofutilizing ICP RIE includes forming fluid fill-holes in said substratefor passage of said fluid.
 4. The method of claim 1 wherein said step ofutilizing ICP RIE includes: providing a first dielectric layer on afirst surface of said substrate; providing a second dielectric layer ona second surface of said substrate; patterning said first dielectriclayer to define an etch pattern for said ICP RIE; and using said seconddielectric layer as an etch-stop layer for said ICP RIE.
 5. The methodof claim 4 wherein said step of utilizing ICP RIE includes forming boresin a direction from said first surface to said etch-stop layer.
 6. Themethod of claim 4 wherein said steps of providing said first and seconddielectric layers include forming thermal oxidation layers.
 7. Themethod of claim 4 further comprising a step of forming a heater on saidsecond surface for manipulation of said fluid.
 8. The method of claim 1wherein said step of utilizing ICP RIE includes forming a structurallyweakened edge portion of said substrate for subsequent removal of saidedge portion.
 9. A method of fabricating a matrix of switching elementscomprising the steps of: providing a waveguide substrate having an arrayof first optical paths and an array of second optical paths whichintersect said first optical paths at trenches, said arrays being on awaveguide surface of said waveguide substrate; forming a heater andfluid feed arrangement such that liquid is selectively supplied to andremoved from intersections of said first and second optical paths,including employing ICP RIE techniques to form said arrangement.
 10. Themethod of claim 9 wherein said step of employing ICP RIE techniquesincludes etching fluid fill-holes.
 11. The method of claim 10 whereinsaid step of etching fluid fill-holes includes etching passages througha heater substrate having a pattern of heaters that has a correspondenceto a pattern of said intersections of said first and second opticalpaths.
 12. The method of claim 9 wherein said step of providing saidwaveguide substrate includes forming an array of waveguides to definesaid first and second optical paths.
 13. The method of claim 11 furthercomprising forming an etch-stop layer on one side of said heatersubstrate and forming a patterned layer on the opposite side of saidheater substrate, said patterned layer defining locations of said fluidfill-holes during said etching of said fluid fill-holes.
 14. The methodof claim 13 wherein said etching step forms passages through said heatersubstrate in the direction of said etch-stop layer.
 15. The method ofclaim 11 further comprising a step of bonding said heater substrate tosaid waveguide substrate.
 16. The method of claim 11 further comprisinga step of utilizing said ICP RIE techniques to define a structurallyweakened edge portion of said heater substrate.